All cells contain mechanisms by which proteins are targeted to different cellular compartments. Co-translational targeting of proteins to cell membranes is mediated by the signal recognition particle (SRP), which recognizes the amino-terminal signal sequence of the nascent polypeptide and targets the translating ribosome to a membrane receptor. Structural and functional elements of the SRP ribonucleoprotein are highly phylogenetically conserved. One protein component of the particle, SRP54 or Ffh (in prokaryotes), is of particular interest because it provides specific recognition of the hydrophobic signal peptide and is also a GTPase which interacts directly with the membrane receptor. GTPases are a ubiquitous family of proteins which use the binding and hydrolysis of GTP to elicit various cellular functions - well studied GTPases are involved in translation, cell regulation, and signal transduction. This proposal is directed towards detailed protein structural understanding of the NG GTPase domain of Ffh. The results will contribute to our understanding of the role of this specific GTPase in the SRP pathway, and, more generally, give insight into how different GTPases build on a common core protein structure to elicit different cellular function. The project has three crystallographic objectives. To obtain highly refined ultra-high (1.0 Angstroms) resolution structural models of the apo- and GDP-bound NG GTPase from crystals now in hand. To improve existing MgGDP-form crystals so that we can describe that structure at a similar level of detail. And, to crystallize the MgGTP-bound NG GTPase using nonhydrolyzable nucleotide analogs or GTP, and to solve its crystal structure as well. The structural data, spanning the ligand states of the GTPase, will provide the basis for detailed and accurate analysis of the interaction between the protein, water, magnesium, and nucleotide. They should also reveal whether structural phenomena not resolved in structures at lower resolution - deviations of the protein stereochemistry, packing imperfections, and conformational substrates - may be relevant to GTPase function. The analyses will address the chemistry of the GTPase and the protein design principles which facilitate mobilization of different structural motifs during the cycle of GTP binding and hydrolysis.